Characterization of microporous materials in terms of surface areas, pore sizes and pore size distributions, is a challenging area of research. The need for improved thin film characterization techniques has become increasingly important for understanding the effects of synthesis techniques as well as the ability to describe the porous properties of membranes and chemical sensors.
Micropores are a unique class of pores which, due to their atomic sizes, have unusual properties. This small size makes microporous materials notoriously difficult to characterize. However, this small size also makes these materials enormously useful for various applications such as gas separation membranes.
Methods to characterize microporous materials have not kept up with the surge in research to produce microporous membranes. The research of thin film properties has always been elusive, especially in the area of microporous materials, since the sensitivity of instrumentation is not conducive to measuring the volume properties of the small pores.
Characterization of thin-film properties is important to industries involved in research or development of membranes, optical coatings, barrier coatings for semiconductors, barrier coatings for preservation, and other thin film technologies where porous microstructure affects the performance characteristics of the materials.
The varied uses of porous materials, have led to a myriad of classifications. Some are based on a physical dimension of the pores, while others are based on the separation properties. The IUPAC (International Union of Pure and Applied Chemists) pore size designations, shown below, will be used herein:
pores of diameter &lt;20 .ANG. are considered micropores; PA1 pores of diameter &gt;20 .ANG. and &lt;500 .ANG. are considered mesopores; and PA1 pores of diameter &gt;500 .ANG. are considered macropores.
Although the prior art has exploded with references to microporous thin films, there still remain relatively few techniques to characterize these films. The most popular are permeability, ellipsometry and gas adsorption.
The most common technique to characterize microporous membranes involves measuring gas permeability through a supported membrane. In a variation of the permeability technique, a microporous membrane is equilibrated with known pressure of a condensable vapor, and the permeability coefficient of an incondensable gas through the membrane is measured. A significant drawback to this technique is that is may take weeks or months to obtain sufficient data to calculate pore size distribution, due to equilibration times.
Ellipsometry is commonly used in conjunction with other characterization techniques to gather complementary data such as film thickness. In one technique, the pores are initially filled with adsorbates of various sizes to obtain a total pore volume. Then the films are equilibrated at some relative pressure (in flowing gas), and the pore volumes are determined using the Lorentz-Lorenz relation. Unfortunately, the calculations are only approximate, since the Lorentz-Lorenz relation used does not include the effects of shape anisotropy on the index, or quantum mechanical effects which may be present for structures of near-atomic dimensions. Additionally, reproducible results were only obtained on hydroxylated surfaces and hydroxillation took from several hours to days to achieve.
Gas adsorption techniques have been extensively used for surface area and pore size analysis of materials. One of the limitations of this technique is that the surface area/pore volume of the material must be large enough for the sensitivity of the instrument, which for commercially available instruments (e.g., Micromeritics ASAP 2000) is typically a surface area of &gt;10 m.sup.2. Adaptions of bulk instrumentation have been attempted to increase this range. In general, this is not enough sensitivity to probe thin films. For example, microporous silicates may have nitrogen surface area as low 1 m.sup.2 /g in bulk form. For a typical 1 .mu.m film of 10% accessible porosity, a film area of &gt;100 cm.sup.2 would be required for standard analysis. Nonetheless, bulk methods have been attempted to characterize porous films, and only illustrate the need for reliable thin film characterization. The results showed an array of inconsistent differences between surface areas and pore radii of the bulk and supported sample, with the bulk values often higher.
There is a need for a method for overcoming the inherent limitations of existing measurement methods. The present invention provides the use of more than one acoustic sensor, each acoustic sensor being differently configured. Using this technique, the contribution of the shear modulus of the sensor itself can be ascertained and the measurement corrected accordingly.